powerpc/64s/idle: avoid POWER9 DD1 and DD2.0 PMU workaround on DD2.1

DD2.1 does not have to save MMCR0 for all state-loss idle states,
only after deep idle states (like other PMU registers).

Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>